This invention relates to a method of purifying a gas flow. More particularly, this invention relates to a method and apparatus for purifying a gas flow from a reactor performing solid phase condensation of a polyester resin.
Methods and arrangements of this type are known from U.S. Pat. Nos. 5,547,652 or 5,612,011. These deal with the purification of the process exhaust gases by means of a catalyzer (in contrast to purification by gas washing) or, in other words, with the afterburning (oxidation) of impurities. It goes without saying that, for this reason, the ratio of oxygen to impurities is of the greatest importance. In this regard, two considerations were hitherto decisive: On the one hand, one assumed that a minimal stoichiometric amount of oxygen was necessary for complete oxidation of the impurities. On the other hand, it was thought that, in light of this requirement, the monitoring of the amount of oxygen was the best method for keeping the purification process under control.
Over time, it was found that in spite of the addition of a stoichiometric amount of oxygen, complete oxidation did not occur, so that the first-mentioned patent still allowed at least 10 ppm of nonburned oxygen. This also proved to be non-optimal with regard to the purification effect, which was the reason that the excess of oxygen was increased yet again. In the results of the second patent, an even higher amount of nonburned oxygen of 250 ppm had to be tolerated; however, it was thought that an optimal solution had been provided.
Both of the named patents relate to the purification of process gases during the production of aromatic polyesters. Naturally, gas purification is of particular interest in aromatic compounds and especially in polyesters; still, it is obvious that such purification methods may also be used with other gases and impurities, e.g., polyamides like aromatic PA or also for the purification of exhaust gases during the processing of natural substances, such as, the roasting of coffee or cocoa.
The present invention now goes in the opposite direction. This basically means that instead of placing the amount of oxygen in the foreground, the amount of the residual impurities is taken as the measuring stick or rather, the ratio of oxygen to residual impurities. This first step of the invention subsequently resulted in surprising findings. On the one hand, tests by the applicant showed that-in direct contradiction to previous observations by the specialists-stoichiometric or hyper-stoichiometric oxygen amounts do not lead to optimal purification and that the previous measures not only necessitate a large amount of oxygen (if one does not simply use air), but, rather, the entire system became oversized and too expensive. On the other hand, it was proven that the previously used measuring principle was based on incorrect assumptions and thereby finally delivered inexact results.
Briefly, the invention provides a method comprising the steps of directing a flow of an exhaust gas from a reactor performing a solid phase condensation of a polyester resin through a first line to a catalyzer. In addition, a flow of gas containing a hypo-stoichiometric amount of oxygen is delivered into the flow of exhaust gas at a predetermined point upstream of the catalyzer for oxidizing impurities in the exhaust gas within the catalyzer. In accordance with the invention, the exhaust gas is quantitatively analyzer with a lambda probe and a signal is emitted that corresponds to a measurement of the purity of the exhaust gas. The lambda probe is disposed in one of the catalyzer and the flow of exhaust gas downstream of the predetermined point. The amount of oxygen-containing gas delivered into the exhaust gas flow is controlled in dependence on a deviation of the emitted signal from a set signal. The invention is further directed to an apparatus comprising a reactor for solid phase condensation of a polyester resin, an exhaust line extending from the reactor for removing exhaust gas from the reactor, a catalyzer for receiving a flow of exhaust gas from the exhaust line and a second line connected to the first line for delivering a flow of oxygen-containing gas into the flow of exhaust gas in the first line upstream of the catalyzer for oxidizing impurities in the exhaust gas within the catalyzer. In accordance with the invention, a lambda probe is provided for quantitatively analyzing the exhaust gas and emitting a signal corresponding to a measurement of the purity of the exhaust gas. This lambda probe is disposed in one of the catalyzer and the first line downstream of the second line. In addition, a processor is provided for receiving the signal from the probe and for controlling the amount of gas delivered from the second line into the first line in dependence on a deviation of the signal from a set signal.
The preferred amount of hypo-stoichiometric oxygen is 98% to  less than 100%, preferably at least 99%, of the stoichiometric amount.
It is completely surprising that even a hypo-stoichiometric amount of oxygen leads to a better result, and the theoretical explanation for this phenomenon does not yet exist. Tests have, however, confirmed this fact; namely, hardly any further measurable residual impurities or residues of nonburned oxygen are being determined. Naturally, it is difficult to still monitor such small residues at all. It has been shown thatxe2x80x94if one actually wishes to carry the improvement with regard to purification to the extremexe2x80x94the customary oxygen sensor delivers only imperfect results. For this reason, according to the invention, one plans to use a lambda probe. It is obvious, in this regard, that such a lambda probe enables better monitoring also in the case of the previously known method, when one utilizes stoichiometric or hyper-stoichiometric oxygen amounts, and that it is, however, particularly advantageous in those cases where the amounts to be monitored are especially small.
A lambda probe (according to R,xc3x6mpps Chemical Lexicon, one writes xe2x80x9cxcex9xe2x80x9d as the symbol for molar electrolyte conductibility) represents two noble-metal films, such as Pt, on a fixed electrolyte with connected electrodes. It is striking that this design is very similar to the catalyzer to be monitored. For this reason, it is completely possible within the framework of the invention to use the catalyzer itself as the lambda probe-like measuring head, wherein the monitoring signal must naturally be correspondingly filtered out, decoupled, or demodulated.
Regardless of how and where the lambda probe is inserted, an essential aspect of its usage is that only a direct measurement of the involved elements or electrolytes, instead of the hitherto indirect measurement, is now possible, wherein it must be added that the lambda probe exhibits particularly advantageous characteristic curves for an exact measurement. That it was previously not used had probably to do with the fact that one was so influenced by the stoichiometric or hyper-stoichiometric oxygen amount that it was thought that the nonburned oxygen must be measured. With the lambda probe, one is, on the other hand, also freer with regard to performing the method, since when the method according to the invention is used for processes other than the polycondensation of synthetic resins, an inert gas is possibly not required at all, wherein the measurement by the oxygen extant in the process would be further impaired in the case of the previous measurement method.
In any case, as is known, one can, however, use an inert gas as an impure gas, which is preferably at least partially recycled back into the reactor following purification.
To render the simple monitoring, which can be performed by the service personnel, into a quickly responding regulatory measure or at least enable such, it is certainly preferable that the monitoring occurs by means of at least one lambda probe in front of the catalyzer and/or that the monitoring occurs by means of at least one lambda proble on the catalyzer. Naturally, the monitoring of the final results of purification, that is, after the catalyzer, is of particular interest; however, such monitoring can also be additionally included in the regulatory measure.